1. Field of the Invention
The present invention relates to a rotor hub to which a recording disk is fixed, a motor which includes the rotor hub, and a recording disk driving device which includes the motor.
2. Description of the Related Art
A recording disk drive, such as a hard disk drive (HDD), is conventionally furnished with a spindle motor for rotationally driving a recording disk. Such spindle motors will be referred to simply as “motors” hereinafter. The recording disk is attached to a rotor hub of the motor. The recording disk includes a center opening, and the rotor hub includes a convex portion. The recording disk is attached to the rotor hub such that a periphery of the center opening of the recording disk is engaged with the convex portion, and is pressed to the rotor hub by, for example, a clamper or a fixing screw. As a result, the recording disk is fixed to the rotor hub with a bottom surface thereof abutted against a disk placing surface of the rotor hub.
Recently, the HDD has been installed in a mobile music player, and the demand for an HDD having a small dimension and a large storage capacity is increasing. In order to enlarge the storage capacity of the recording disk without enlarging the dimension of the recording disk, it is preferable to utilize a storage area of a bottom surface of the recording disk more effectively.
However, enlarging the effective storage area of the bottom surface of the recording disk results in reducing an abutting area between the recording disk and the rotor hub. Therefore, it is difficult to securely hold the recording disk on the rotor hub. For example, the recording disk may be bent by pressure applied by, for example, the damper if the abutting area between the recording disk and the rotor hub is reduced. As a result, an outer circumferential portion of the disk may be curved or a portion of the recording disk may be shifted relative to the rotor hub. Such a recording disk which is curved or bent may cause a reading/writing error of data.
An inner circumferential side of the disk placing surface provided on the rotor hub generally includes a relief portion (so-called undercut portion), generally as a downwardly extending convex portion, formed when a cutting tool changes its orientation during the cutting of the rotor hub. Therefore, the abutting area between the recording disk and the disk placing surface is further reduced, and it becomes more difficult to securely hold the recording disk on the rotor hub. Particularly, the impact which the relief portion imposes on the interface between the recording disk and the rotor hub is significant when the recording disk has a small dimension, such as a 1.8-inch type disk.
The relief portion may be diminished by using an extremely sharp cutting tool whose edge has an extremely small radius of curvature. However, such a cutting tool is not very durable, such that the appropriate productivity of the rotor hub cannot be maintained. In addition, the cost of manufacturing the rotor hub increases because commercial cutting tools do not have sufficient sharpness for this purpose.